1. Field of the Invention
The present invention relates to a data transmission apparatus including a plurality of stations linked in the form of multidrop and more specifically to a data transmission apparatus including a plurality of remote repeaters as the regenerating repeaters for data transmission between respective stations.
2. Description of the Prior Art
A block diagram indicating a structure of the conventional transmission apparatus, for example, of the U.S. Pat. No. 4,700,342 is shown in FIG. 1. In this figure, a first and a second signal lines 11 and 21, each consisting generally of twisted pair cable or coaxial cable, are used as the transmission buses for signal transmission. Provided at both ends of the first and second transmission lines 11 and 21 are terminating resistors 12, 13, 22, 23 for transmission line impedance matching. The first and second signal lines 11 and 21 are respectively linked to the transmitting and receiving stations 14, 15, 24 and 25. These first and second signal lines are further linked respectively to a first and a second remote repeaters 16, 26, which are connected through a remote repeater cable 31, consisting of twisted pair cable or coaxial cable.
A remote repeater line 30 is formed by the first and second remote repeaters 16 and 26 and the cable 31.
A block diagram indicating a structure of the first remote repeater 16 of FIG. 1 is shown in FIG. 2. In this figure, the signal sent from the first signal line 11 and cable 31 is received by first and second receivers 41, 51. The signal received by the first and second receivers 41 and 51 is waveform-shaped by first and second waveform shaping circuits 42, 52. A carrier is detected from the signal output from the first and second receivers 41, 51 by first and second carrier detecting circuits 43, 53. Detection sequence of carrier output from the first and second carrier detecting circuits 43, 53 is decided by a decision circuit 44. Outputs of the first and second waveform shaping circuits 42, 52 and decision circuit 44 are input to first and second AND circuits 45, 55. Outputs of the first and second AND circuits 45 and 55 are respectively output to the first signal line 11 and cable 31 by first and second drivers 46, 56.
The first remote repeater 16 has the structure designed for bothway transmission using the first and second receivers 41, 51 and the first and second drivers 46, 56.
The second remote repeater 26 has the same structure as the first remote repeater 16.
Next, operation for sending the signal from the station 14 will be explained hereunder.
When the signal is first transmitted to the first signal line 11 from the station 14, the first receiver 41 of the first remote repeater 16 receives the carrier sent from the first signal line 11. Thereby the first carrier detecting circuit 43 turns ON and the decision circuit 44 sends a signal to open the first AND circuit 45, upon reception of the carrier from the first signal line 11. Therefore, the signal received by the first receiver 41 is output, upon shaping by the first waveform shaping circuit 42, to the cable 31 from the first driver 46 through the first AND circuit 45.
The second remote repeater 26 having received the signal from the cable 31 shapes waveform of the signal received from the cable 31 and then outputs the signal to the second signal line 21 like the first remote repeater 16. Therefore, the stations 24, 25 can receive the signal from the station 14.
Such operation conceives a problem that the signal output from the first driver 46 of the first remote repeater 16 is received in direct by the second receiver 51 and thereby the second carrier detecting circuit 53 turns ON.
However, the decision circuit 44 detects which circuit of the first and second carrier detecting circuits 43, 53 becomes active precedingly and decides an output. Accordingly, the decision circuit 44 does not disturb the signal waveform of the first signal line 11 by operating the second driver 56. Namely, as shown in the time chart of FIG. 3, the first AND circuit 45 opens and the second AND circuit 55 closes. Next, when the station 14 suspends transmission of signal, the first carrier detecting circuit 43 turns OFF and the decision circuit 44 closes the first AND circuit 45, waiting for detection of the next carrier.
Moreover, operation of the first remote repeater 16 in transmission of signal from the station 24 will be explained hereunder.
First, the second receiver 51 of the first remote repeater 16 receives the carrier from cable 31, the second carrier detecting circuit 53 turns ON and decision circuit 44 confirms reception of carrier from the cable 31 and sends an output to open the second AND circuit 55. Thereby, the signal received by the second receiver 51 is once shaped by the second waveform shaping circuit 52 and is output to the first signal line 11 from the second driver 56 passing through the second AND circuit 55. Terefore, the stations 14, 15 are capable of receiving the signal from the station 24.
The transmission apparatus of the prior art is thus constituted as explained above. Namely, the first and second signal lines 11 and 21 are linked with the remote repeater line 30 for one channel. Therefore, such transmission apparatus results in a problem that transmission becomes impossible if the outdoor cable 31 laid for long distance is broken, for example, by construction work.